U.S. patent number 4,544,237 [Application Number 06/334,681] was granted by the patent office on 1985-10-01 for high efficiency optical tank for two-color liquid crystal light valve image projection with color selective prepolarization.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Ralph J. Gagnon.
United States Patent |
4,544,237 |
Gagnon |
October 1, 1985 |
High efficiency optical tank for two-color liquid crystal light
valve image projection with color selective prepolarization
Abstract
The invention provides a two-channel liquid crystal light valve
image projection system with prepolarization using an oil coupled
optical arrangement. A prepolarizing beam splitter is mounted the
container in optical alignment with a first surface for splitting
and prepolarizing light from a source into first and second beams
having first and second polarization states respectively. First and
second dichroic separators are mounted within the container for
extracting light of a first color from the first beam and light of
a second color from the second beam respectively. The separator
outputs are recombined by the prepolarizing prism into a single
beam which is directed to a second polarizing beam splitter mounted
within the container. The second beam splitter splits the single
beam and directs light of the first and second polarizations
through the second and third apertures, respectively. Liquid
crystal light valves mounted at the second and third apertures
modulate the polarization state of the exiting light and return it
to the second beam splitter to be directed through the output
aperture to projection optics for display.
Inventors: |
Gagnon; Ralph J. (Chico,
CA) |
Assignee: |
Hughes Aircraft Company (El
Segundo, CA)
|
Family
ID: |
23308313 |
Appl.
No.: |
06/334,681 |
Filed: |
December 28, 1981 |
Current U.S.
Class: |
349/9;
348/E9.027; 349/8; 353/31; 359/487.04 |
Current CPC
Class: |
H04N
9/3167 (20130101) |
Current International
Class: |
H04N
9/31 (20060101); G02F 001/13 (); G03B 021/00 () |
Field of
Search: |
;350/342,345
;353/31,34,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Hong et al., Application of the Liquid Crystal Light Valve to a
Large Screen Display..
|
Primary Examiner: Corbin; John K.
Assistant Examiner: Lewis; David
Attorney, Agent or Firm: Benman, Jr.; W. J. Durant; S. C.
Karambelas; A. W.
Claims
What is claimed is:
1. A unitary arrangement comprising:
a first planar polarizing beam splitter for splitting and
prepolarizing light from a source into first and second beams
having first and second polarization states, respectively;
a first dichroic separator for extracting light of a first color
from said first beam;
a second dichroic separator for extracting light of a second color
from said second beam;
means for combining the outputs of said first and second dichroic
separators into a single beam; and
a second polarizing beam splitter for splitting and polarizing the
single beam and directing light of the first color to a first
surface and light of the second color to a second surface, wherein
the first and second polarizing beamsplitters have transverse axes
and each beamsplitter is mounted so that the beamsplitter
transverse axes are oriented at a relative 90.degree. angle.
2. The unitary arrangement of claim 1 including mirror means for
directing the outputs of said first and second separators to said
means for combining the outputs of said first and second
separators.
3. A multi-color image projection system comprising:
first means for providing a source of light energy; and
a unitary tank including a first polarizing beamsplitter for
splitting and prepolarizing light energy from said first means into
first and second means having first and second polarization states
respectively, a first dichroic separator for extracting light
energy of a first color from said first beam, a second dichroic
separator for extracting light energy of a second color from said
second beam, means for combining the outputs of said first and
second dichroic separators into a single beam and a second
polarizing beamsplitter for directing light energy of the first
color to a first light valve, light energy of the second color to a
second light valve and light energy from said first and second
light valves to a means for projection, wherein the first and
second polarizing beamsplitters have transverse axes and each
beamsplitter is mounted so that the beamsplitter transverse axes
are oriented at a relative 90.degree. angle.
4. The unitary arrangement of claim 3 wherein the beamsplitters,
separators and means for combining are coupled through oil.
5. The multicolor image projection system of claim 3 including
first and second cathode ray tubes for providing an input image for
said first and second liquid crystal light valves respectively.
6. The multi-color image projection system of claim 3 wherein the
beamsplitters, separators and means for combining are coupled
through oil.
7. The multi-color image projection system of claim 3 including
mirror means for directing the outputs of said first and second
separators to said means for combining the outputs of said first
and second separators.
8. In an optical system including means for providing light energy,
first and second liquid crystal light valves, first and second
cathode ray tubes for use with said first and second liquid crystal
light valves, respectively, and a means for projection, an improved
optical arrangement comprising:
a container having a plurality of surfaces joined together to form
a chamber suitable for holding fluid, including a first surface for
providing an input aperture, second and third surfaces each
providing an output aperture for polarized and an input aperture
for modulated light, energy and a fourth surface for providing an
output aperture;
a prepolarizing beamsplitter mounted within said chamber for
splitting and prepolarizing light energy from the source into first
and second beams having first and second polarization states
respectively;
a first dichroic separator mounted within said chamber for
extracting light energy of a first color from said first beam and a
second dichroic separator for extracting light energy of a second
color from said second beam;
means mounted in said chamber for combining the output of the first
and second separators to form a single beam; and
a main polarizing beamsplitter mounted in said chamber for
directing light of a first color and polarization through the
second aperture to the first liquid crystal light valve and light
energy of a second color and polarization through the third
aperture to the second liquid crystal light valve and directing
intensity modulated light energy through said fourth aperture to a
means for projection, wherein said prepolarizing beamsplitter and
said main polarizer have transverse axes and each beamsplitter is
mounted so that the beamsplitter transverse axes are oriented at a
relative 90.degree. angle.
9. The optical system of claim 8 wherein the prepolarizing
beamsplitter and the main polarizing beamsplitter are substantially
identical.
10. A unitary arrangement comprising:
a first polarizing beamsplitter for substantially splitting and
polarizing light from a source into first and second beams having S
and P polarization states, respectively;
a first dichroic separator for substantially extracting red light
from said first beam;
a second dichroic separator for substantially extracting green
light from said second beam;
means for combining the outputs of said first and second dichroic
separators into a single beam; and
a second polarizing beamsplitter for polarizing the single beam and
directing red light to a first surface and green light to a second
surface, wherein the first and second polarizing beamsplitters have
transverse axes and each beamsplitter is mounted so that the
beamsplitter transverse axes are oriented at a relative 90.degree.
angle.
11. The unitary arrangement of claim 10 wherein the beamsplitters,
separators and means for combining are coupled through oil.
12. The unitary arrangement of claim 10 including mirror means for
directing the outputs of said first and second separators to said
means for combining the outputs of said first and second
separators.
13. The unitary arrangement of claim 10 wherein the first and
second polarizing beamsplitters are substantially identical.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention related to liquid crystal light valve projectors.
Specifically, this invention relates two color liquid crystal light
valve projectors with oil coupled dichroics.
While the present invention will be described herein with reference
to particular applications, it is to be understood that the
invention is not limited thereto. Those having ordinary skill in
the art and access to the teachings of this invention will
recognize additional applications within the scope thereof.
2. Description of the Prior Art
The development of the liquid crystal light valve has opened the
door to substantial progress in the state of the art of high
quality large screen projectors. The reflective mode liquid crystal
light valve is a thin film, multilayer structure comprising a
liquid crystal layer, a dielectric mirror, a light blocking layer,
and a photoresponsive layer sandwiched between two transparent
electrodes. A polarized projection beam is directed through the
liquid crystal layer onto the dielectric mirror. An input image of
low intensity light, such as that generated by a cathode ray tube,
is applied to the photoresponsive layer thereby switching the
electric field across the electrodes from the photoresponsive layer
onto the liquid crystal layer to activate the liquid crystal.
Linearly polarized projection light passing through the liquid
crystal layer and reflecting from the dielectric mirrors is
polarization-modulated in accordance with information incident on
the photoconductor. Therefore, if a complex distribution of light,
for example, a high resolution input image, is focused onto the
photoconductor surface, the device converts the image into a
replica which can be projected with magnification to produce a high
brightness image on a viewing screen. U.S. Pat. No. 4,019,807
issued to D. D. Boswell et al on Apr. 26, 1977 discloses such a
high performance reflective mode liquid crystal light valve.
A graphics display projector using a liquid crystal light valve of
the above-type is described in an article entitled "Application of
the Liquid Crystal Light Valve to a Large Screen Graphics Display",
published in the 1979 Society for Information Display, (SID),
International Symposium, Digest of Technical Papers, May 1979, pp.
22-23. This display system, a type with which the present invention
is particularly but not exclusively concerned, projects a large
scale image having yellow-white characters on a dark blue
background. The system includes a cathode ray tube (CRT) which
provides input imagery; projection optics which provide the bright
collimated output beam and necessary light polarization; and the
liquid crystal light valve which interfaces the input and output
functions.
The system uses a powerful light source such as a xenon arc lamp to
illuminate the liquid crystal light valve through collimating and
polarizing optics. Light emitted from the xenon arc lamp is
transmitted to a polarizing main prism where it is separated into
`S` and `P` components. The `P` component passes through the prism
while the `S` component is reflected toward the light valve.
Information displayed by cathode ray tube is transferred by fiber
optics to one side of the light valve which changes the
polarization state from `S` to `P`. The light is then transmitted
through the prism and imaged on a screen by projection lens. In
this capacity, the main prism functions as an analyzer, converting
modulations of polarization to modulations of brightness or
intensity.
The quality of the projected image is generally a function of
brightness, resolution and contrast. Image quality can generally be
improved by placing a prepolarizing prism in the optical path in
front of the main polarizing prism. The prepolarizing prism is
somewhat effective in overcoming the deficiencies in the main
polarizing prism. That is, since the main polarizing prism is not
100% effective in transmitting light of one polarization and
reflecting light of another, light of an undesirable polarization
may reach the light valve and be modulated and reflected back
through the main prism onto the projection lens. This often results
in distortions of color and/or reductions in contrast and
resolution.
Since the prepolarizing prism may, for reasons of cost, be of the
same design as the main prism, it would typically have similar
reflectance and transmittance characteristics. However, when the
two prisms are used in combination, the additive effect is such as
to greatly improve the quality of the projected image. The
prepolarizing prism substantially removes light of one polarization
from the beam which illuminates the main prism. The main prism then
acts on the beam to substantially remove the residual light of the
undesirable polarization.
In some applications, it is desirable to use a second liquid
crystal light valve for enhanced information displaying capability
and versatility. In this application, the use of the
prepolarization prism becomes problematic insofar as the light
valve would require light of the polarization that would otherwise
be removed by the prepolarizing prism. As a result, the use of a
second light valve has forced a compromise in the quality of the
projected image.
This problem was addressed by the Applicant and Roy Cedarstrom and
Ralph Gagnon in a copending application entitled "Two-Color Liquid
Crystal Light Valve Image Projections System with Single
Prepolarizer" Ser. No. 06/334,679, now U.S. Pat. No. 4,500,872. It
provides a color selective prepolarization of the light incident
upon two or more light valves. This is accomplished by use of a
prepolarizing prism which acts on light from a light source to
direct light of a first polarization to a first dichroic separator
and light of a second polarization to a second dichroic separator.
The resulting beams are recombined in a dichroic adder prior to
being applied to a second polarizing prism. The second polarizing
prism directs light of a first color and polarization to a first
light valve and light of a second polarization to a second light
valve in the conventional manner.
This system, though effective, is bulky insofar as each prism is
essentially a small tank with one or more optical thin film layers
immersed in glass or oil. The system of the copending application
requires air coupling to the immersed beam splitters. The resulting
arrangement requires much attention to the proper alignment of the
components while offering less than optimal performance. It is
known that the coupling of the beam splitters through oil would
improve the performance of this system.
SUMMARY OF THE INVENTION
The present invention provides a two-channel color selective
prepolarization in oil with oil coupled dichroics. The invention
includes a prepolarizing beam splitter for splitting and
prepolarizing light from the source into first and second beams
having first and second polarization states, respectively. First
and second dichroic separators are included for extracting light of
a first color from the first beam and light of a second color from
the second beam respectively. The resultant output of each
separator is a collimated polarized monochromatic beam. The
separator outputs are recombined by a beam combiner into a single
beam which is directed to a second beamsplitter (main prism). The
main prism splits the single beam and directs light of the first
and second polarizations through the second and third apertures,
respectively. Liquid crystal light valves mounted at the second and
third apertures modulate the polarization state of the exiting
light and return it to the main prism to be directed to the fourth
aperture. The system thus provides two-channel prepolarization by
way of an oil coupled optical arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
The FIGURE is a perspective view of a diagramatic representation of
a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in the FIGURE, the invention 10 includes a container 12
made of metal, plastic, glass or other suitably rigid material. In
the preferred embodiment, the container is glass. The container 12
is dimensioned to provide the most compact unit with regard to the
requirements that the polarizers be oriented at a predetermined
angle relative to incident light. The container is hollow having
walls on the order of 1/4 inches thick. The exterior dimensions of
the container are determined with regard to the dimensions of the
associate optical components discussed below. In the preferred
embodiment, the container is filled with optical grade oil having
an index of refraction of 1.622. It is understood that glass or
oils of other indices of refraction may be used without departing
form the scope of this invention. Note that the use of oil or glass
of another index may require a change in the below described design
of optical components. The container 12 has side surfaces 14 and
16, rear surfaces 18 and 20, top surface 22, front surface 24, and
lower surfaces 26, 28, 30 and 32. The surfaces 14-32 may include
milled grooves, posts or special adhesives (not shown) which are
effective in maintaining the optical components in proper alignment
and effectively sealing the spaces therebetween. As described more
fully below, surfaces 14, 16, 24 and 32 are transparent and provide
apertures through which light enters and leaves the system.
The surface 32 provides an input aperture. In the preferred
embodiment it is a glass sharp-cut Schott GG47S filter having an
index of refraction of 1.54. The surface 32 is coated to reflect
ultraviolet light and prevent overheating. In the preferred
embodiment, the coating is constructed at a wavelength (or optical
thickness) .lambda.=390 nm. It includes a first layer of depth
d=1.49 quarter waves and index of refraction n=1.92. This first
layer is topped with 10 sets of layers of depth d=0.5 quarter waves
and index n=1.46, a second layer of depth d=1 quarter wave and
n=2.32 and a third layer of depth d=0.5 quarter wave and index
n=1.46. These sets of layers are topped with a final layer of depth
d=1.49 quarter waves and index n=1.55. This coating also serves as
an anti-reflection coating for red and green wavelengths.
A prepolarizing beam splitter 34 is mounted within the container 12
so that its transverse axis lies between surfaces 14 and 16. It is
disposed in optical alignment with the ultraviolet filter at
surface 32. The prepolarizing beam splitter 34 is constructed of
glass or optical grade fused silica having a birefringence less
than or equal to 6 nm/cm and an index n=1.62. The prepolarizer 34
is a plate which, in the preferred embodiment, has dimensions of
7.6".times.2.9".times.1/8". The prepolarizer 34 has an upper
portion 36 and a lower portion 38. The lower portion 38 serves as a
prepolarizing beam splitter. The upper portion 36 serves as a beam
combiner. The prepolarizing beam splitter 34 has a thin film
coating which, in the preferred embodiment, is constructed at an
optical thickness .lambda.=500 nm. The coating includes a first
layer of depth d=1.557 quarter waves and index n=2.05; a second
layer of depth d=0.994 quarter waves and index n=1.35; five sets of
layers each set having a first layer of depth d=1.157 quarter waves
and index n=2.32 and a second layer of depth d=1.988 quarter waves
and index n=1.35; followed by a layer having a depth d=1.157
quarter waves and an index n=2.32; followed by a layer of depth
d=0.994 quarter waves and an index n=1.35; and a final layer of
depth d=1.557 quarter waves and an index n=2.05.
The prepolarizing beam splitter 34 is mounted so that as its
transverse axis is horizontal to the plane of the input aperture
32. In the preferred embodiment, it was found than an orientation
of the longitudinal axis of 48.degree. relative to incident light
provides the best compromise between contrast, size and cost. It
should be noted, however, that other orientations may be employed
without departing from the scope of the invention. A change in
orientation will typically require a change in the design of the
container 12 as well as the coating on the prepolarizer 34. Such a
change is nonetheless within the scope of the invention.
A red pass filter 35 is mounted between the prepolarizer 34 and the
intersection of surfaces 28 and 30. A green pass filter 37 is
mounted between the prepolarizer 34 and the intersection of
surfaces 18 and 20. Each filter is made of an optical grade of
fused silica having an index of refraction n=1.62 and a
birefringence less than or equal to 6 nm/cm. In the illustrative
embodiment, each has dimensions 2.9".times.2.6".times.1/8". The red
filter 35 is coated to pass red light and reflect light of other
colors. The coating includes a first layer of depth d=1.228 quarter
waves and index n=2.32; 13 sets of layers each set having a first
layer of depth d=0.5 quarter waves and index n=1.46; a second layer
of depth d=1 quarter waves and index n=2.32 and a third layer of
depth d=0.5 quarter waves and index n=1.46; and a final layer of
depth d=1.228 quarter waves and index n=2.32. The construction is
at an optical thickness .lambda.=492 nm.
Similarly, the green filter 37 is coated to pass green light and
reflect light of other colors. Its coating includes a first and
last layer of depth d=0.85 quarter waves and index n=2.32 between
which 15 sets of layers are sandwiched, each set including a first
layer of depth d=0.5 quarter waves and index n=2.05, a second layer
of depth d=1.0 quarter waves and index n=1.6, and a third layer of
depth d=0.5 quarter waves and index n=2.05. This construction is at
an optical thickness .lambda.=640 nm.
A first mirror 40 is disposed on the interior of the surface 28.
The mirror 40 is of a conventional construction with birefringence
less than 6 nm/cm. No optical thin film coatings are required. The
mirror may be constructed of Schott F2 glass of index of refraction
of 1.62. The mirror dimensions are 4.3".times.2.9".times.1/8".
A second mirror 42 is mounted on the interior of surface 20. The
mirror 43 is identical to the mirror 40 with the exception that its
dimensions are 3.7".times.2.9".times.1/8". It should be noted that
the relative positioning of the filters 35 and 37 and the mirrors
40 and 42 may be changed so that the filters are in the optical
path after the mirror without departing from the scope of the
invention.
The main polarizer 44 is the second polarizing beamsplitter of this
invention. It is oriented at a twist relative to the prepolarizing
beam splitter 34 such its transverse axis lies between surfaces 22
and 26, perpendicular to the transverse axis of the prepolarizing
beam splitter 34. As a result, the beam-splitting and color
separating plates are perpendicular to a common vertical plane in
the prepolarizing section while the plates in the main polarizer
are perpendicular to a horizontal plane. This results in two
advantages. First, this allows the illumination light to be brought
in on a vertical line from below the prepolarizer 34 thereby
reducing physical awkwardness. Second, it results in improved
polarizing beam splitter performance. According to calculations the
performance improvement significantly obviated the necessity for
trim filters at the light valve and the exist windows. This
performance improvement results from the fact that with most
currently available polarizers, polarization by transmission is
more effective than polarization by reflection. That is, when the
prepolarizer 34 reflects S polarized light through the red filter
35 and transmits P polarized light through the green filter 37,
some P polarized light is also reflected to the red filter 35.
Without the 90.degree. twist, and since for reasons of economy the
main polarizer has the same design as the prepolarizer 34, the main
polarizer would similarly reflect some green P polarized light to
the red light valve. This necessitates the use of a light lowering
red trim filter in front of the red light valve to remove the
reflected green P polarized light. The problem is further
exacerbated by the fact that the transmission of green P polarized
light to green light valve in the off state will result in the
reflection of the green P polarized light back to the main
polarizer 44. Most of this light will pass through the polarizer 44
and return to the illumination system. However, once again some
green P polarized light is reflected to the red light valve by the
beam splitter 44. This light reaches the projection screen and
lowers image contrast.
Since the 90.degree. twist at the main polarizer 44 interchanges
the roles of S and P polarized light, the main polarizer 44 sees
green S and red P polarized light. Since there is no green P
polarized light present at the main polarizer 44, no trim filter is
required at the red light valve. Thus, the system is more efficient
and the displayed image is brighter. In addition, no green P is
projected on the screen and the displayed image has greater
contrast.
The main polarizer 44 is constructed of Schott F2 glass with an
index of refraction of 1.62. The main polarizer 44 is mounted so
that light will be incident on it at an angle of 48.degree.
relative to its longitudinal axis. It has dimensions of
3.4".times.2.5".times.1/4". As mentioned above, the main prism 44
has the same thin film coating as the prepolarizing beam splitter
34.
A source 70 and collimating optics 72 are mounted in optical
alignment with the input surface 32. Liquid crystal light valves 74
and 76 are mounted parallel with surfaces 16 and 24, respectively.
Cathode ray tubes 78 and 80 are mounted in optical alignment with
liquid crystal light valves 74 and 76, respectively.
In operation, the source 70 emits unpolarized light which is
collimated by lens 72 and filtered by UV filter at surface 32. The
filtered collimated unpolarized light is incident on the
prepolarizing beam splitter 34 at an angle of 48.degree.. The S
polarized light is transmitted through the red filter 35 and
reflected by mirror 40 to the upper portion 36 of the prepolarizer
34. The P polarized light is transmitted through a green filter 37
and reflected by mirror 42 to the upper portion 36 of the
prepolarizing beam splitter 34. The prepolarizing beam splitter 34
recombines the beams into a single beam and reflects it to the main
polarizer 44. As discussed above, since the main polarizer 44 has a
traverse axis perpendicular to that of the prepolarizer 34, the
polarization states of the incident light are reversed. The red S
output of the prepolarizer 34 becomes a red P relative to the main
polarizer 44 and is transmitted to the liquid crystal light valve
76. Similarly, the green P component transmitted via the
prepolarizer 34 is reflected as green S by the main polarizer 44 to
the light valve 74. The light valves 74 and 76 modulate the
polarization states of incident light in accordance with writing
light provided by cathode ray tubes 78 and 80 in a conventional
manner. The polarization modulated light is returned to the main
polarizer 44 where modulations of polarization are converted to
modulations of intensity and are transmitted to the projection lens
84. It should be noted that many of the above described optical
coatings were designed and performance evaluated by the Thin Film
computer program provided as a service by the Genesee Company of
Rochester, N.Y.
The present invention has been described with reference to a
particular embodiment in a particular application. It is understood
that other designs of the container may be utilized without
departing substantially from the scope of the present invention. It
is also understood that certain modifications can be made with
regard to the selection of polarizaton components to be filtered by
the red and green filters, respectively. In addition, other
dichroic filters may be utilized without departing from the scope
of the invention. The prepolarizing beam splitter 34 need not be a
unitary beam splitter but may, instead, be two separate
prepolarizing beam splitters. Although the invention of the
preferred embodiment is immersed in an optical grade oil of an
index of refraction of 1.622, glass or oils of other indices may be
chosen in accordance with the particular design of the system 10.
It is anticipated by the appended claims to cover any and all such
modifications.
* * * * *